Drilling mud selection, type, and composition used are a very important part of a drilling process. The drilling mud has been called the “lifeblood” of the drilling operation since it is responsible for the removal of cuttings, lubricity, bore stability, and also prevention of inflow between borehole and formations (Ekunsanmi, 2012). Torque reduction by drilling fluids in directional drilling and extended reach wells is a subject of special importance. Friction and high torque and drag, resulting from drill string and wellbore casing interaction could cause a stuck pipe and even a loss of the well. Even though oil-based muds and synthetic-based muds performance has been proved to provide high temperature stability and produce lower friction and torque values (Sonmez, Veran Kok, & Ozel, 2013), these muds are severely limited due to high costs and environmental regulations (Melton et al., 2000). For this reason, water-based muds are preferred, even though additives are necessary to reach desired levels of performance. These muds are considered environmentally friendly, low cost and not toxic compared to oil-based muds.
To decrease the torque caused by high frictional forces in drilling, lubricants are used in water-based muds. Lubricants can be divided into two types: solid lubricants and liquid lubricants. Solid lubricants work like ball bearings and do not bond to contact surfaces. Solid lubricants performance is independent from the drilling fluid type as there is no interaction between the drilling fluid and the solid lubricant. However, solid lubricants may cause plugging problems. Liquid lubricants form a thick film between surfaces, masking surface roughness and withstands high compressional forces (Sonmez et al., 2013). As liquid lubricants interact with other surface active materials, their performance depends on their concentrations. Diesel oil and crude oil of varying densities have been applied by the industry, but they do not perform well when compared to established commercial lubricants (which are specially made for lowering friction). Liquid lubricants include triglycerides, vegetable based oil, glyceride and polypropylene based lubricants, and fossil fuel blends with chemicals. Liquid lubricants have relatively lower density compared to other compounds in the drilling and oil-water emulsion causing foam, which needs density controlling additives. Furthermore, commercial lubricants could cause toxicity to marine life and biodegradability problems (Melton et al., 2000). Along with lubricants, paraffins, esters aldehydes, acids, and other polymeric compounds are being used as additives for volume of filtration loss in milliliters of water from drilling mud through the mud cake formed at certain conditions (fluid loss), viscosity, gel strength as well as emulsifying agents and corrosion inhibitors. Desired properties of these additives include non-toxicity (or low-toxicity), biodegradability, avoidance of an oily slick on water.
Research has evolved on the use of microorganisms as drilling mud additives. Unextracted Lipids inside microorganisms were found to reduce torque. Experiments were conducted by Dillon, Ngantung, and Echaniz (2014) using oleaginous microorganisms having more than 45-85% of lipids (cell dry weight). Results showed a 65% torque reduction. The reduction is directly proportional to the concentration of lipids inside the cells (microorganisms). The impact of cell addition on torque was caused by lipids but also cellular material. Cell lysis usually occurs to microorganisms due to the extreme pressures at the drill bit. Lipids are now free to lubricate the drill bit, forming a thin layer between the surfaces. These investigators demonstrated that lysing microbes prior to addition to the drilling mud performs better in terms of lubricity compared to adding the whole cells. Previous studies demonstrated that activated sludge generated by wastewater treatment facilities could reduce the friction, fluid loss and impart desired rheological properties to the mud (Parker, 1966). The dried sludge from a treatment facility primarily reduced friction and fluid loss.
Waste activated sludge is generated as a by-product of microbes consuming the organic matter present in sewage water during treatment. It is basically the excess cell growth. Microorganisms with the metabolic capability to accumulate more than 20% of their weight as lipids are considered oleaginous. This class of microbes are capable of accumulating lipids up to 80% cell dry weight (CDW) (Dillon et al., 2014). Sewage sludge generated in wastewater facilities contains 5-10% CDW of extractable lipids (Mondala et al., 2008) (Fortela, 2016). Previous studies have demonstrated that waste activated sludge is capable of accumulating lipids, similar to oleaginous microorganisms, under environmental stress. Enhanced Activated Sludge contains 5-80% CDW of extractable liquids. A common stress condition is a high carbon:nitrogen ratio. Once nitrogen is depleted, microorganisms transform the carbon source to lipid globules for energy storage. Mixed cultures are most cost effective than pure cultures since sterilization is unnecessary (Mondala, 2010). Additionally, millions of tons of waste activated sludge are generated annually. Thus, adding value of this waste by transforming it into a drilling mud additive could not only result in a new renewable and environmentally friendly product for oil exploration, but also contribute to the cost effective management of waste activated sludge. Some wastewater treatment facilities are spending a significant fraction of their budgets managing this wastewater treatment by-product. Enhanced activated sludge (EAS) is waste activated sludge cultured at stressed conditions to trigger lipid accumulation. Glucose as the carbon source in the preparation of EAS results in the synthesis of triglycerides. Approximately 80% of the EAS are observed to be oleaginous microorganisms (Mondala et al., 2012).